Optical aiming devices which have a reticle produced from a light source, such as a light emitting diode (LED) or laser diode, are commonly used for aiming firearms, such as pistols, rifles or shot guns or the like. The reticle is projected onto a semi transparent mirror which reflects the reticle image onto the eye retina of the viewer, i.e. the shooter. Hence the viewer can see both the field of view and the projected image of the reticle simultaneously. The mirror is usually a semi transparent concave mirror which reflects the light from the light source as collimated beams, this enables a parallax free image of the reticle. If only the reticle can be seen, the aiming device is operational. The viewer also perceives the reticle as if the reticle is located at a very remote position from the optical aiming device. The reticle can be dots, rings or other patterns.
One optical aiming device is described in the patent application of US 2002/0078618 A1. The optical sight in the document comprises a light emitting diode which is arranged in working cooperation with a plurality of reticle patterns which can be selectively illuminated. The selective illumination is done by connecting various portions of the reticles patterns to the source of the power supply. By selectively illuminating different reticles, the viewer is said to enable a high accuracy in positioning the reticle elements. No moving parts are used as the illumination of the reticle is done by means of electrically switching between the reticle patterns. The brightness of the image can be adjusted by changing the current supplied to the LED. Further, a feedback line can be connected to adjust the brightness of the LED as a function of the environmental lighting conditions. The document is however silent of how to achieve this.
US 2006/0164704 A1 disclose an optical sight similar to the one described above which uses a laser emitting diode as a light source. The laser diode emits the light onto a semitransparent mirror which reflects the light in the form of a reticle image onto the retina of the viewer. The brightness of the reticle can be changed by manipulating the duty cycle of the signals that is applied to the laser diode. The proposed solution only regulates the standard illumination of the reticle.
A drawback with the above mentioned optical sights is that they do not really consider that the target can be subjected to a first light condition and that the optical sight itself can be subjected to another light condition. The mentioned solutions in the prior art regulates the reticle illumination as a function of the average ambient illumination surrounding the aiming device. The difference of the light conditions at the optical sight and at the target can however severely influence the viewer's perception of the reticle and thus be a major influence of the prospect of aiming and firing an accurate shoot.